This invention relates to a machine and a method for manufacturing composite filters, that is to say, filters comprising two or more filter plugs.
The term “composite filter” means a cigarette filter obtained by joining end-to-end two or more filter plugs having different filtration properties and/or made of different materials.
Document EP1787534, in the name of the same Applicant as this invention, discloses a twin-track machine for manufacturing composite filters. It involves dividing up at least two segments of filter material, supplied to respective reservoirs, to make filter plugs from them.
These filter plugs are transferred along a direction transverse to their longitudinal axes by a train of rotating transfer rollers of known type.
The machine comprises an assembling unit designed to place in axial end-to-end contact at least two plugs obtained from two different segments of filter material to obtain filter groups.
The filter groups are taken up and transferred in pairs by a single rotating member presenting circumferential carriers, each furnished with two flutes connected to suction means.
Each flute receives and accommodates a filter group.
The rotating member releases the filter groups in pairs to a pair of conveyors of a garniture tongue which forms two “filter rods”.
The garniture tongue affords two channels in which the filter rods supplied by the two conveyors are fashioned.
At the garniture tongue, the filter groups are wrapped in a strip of paper material to form two continuous filter rods. The rods feeding out of the garniture tongue are cut simultaneously at a single cutting station by a single cutting element.
The absolute and/or relative speeds of the garniture tongue are governed according to a signal from a sensor located upstream of the cutting station.
The sensor measures the relative phase between the two rods and between one of the two rods and the cutting element.
The expression “relative phase of the filter rods” means the relative distance of two predetermined filter plugs belonging to two different rods along the feed direction. For the rods to be cut correctly, this distance must be equal to a reference distance (at which the two rods are perfectly in phase with each other).
The expression “phase of one of the rods relative to the cutting element” means the relative position of a predetermined plug from one of the two rods relative to the position of the cutting element. For the rods to be cut correctly, this distance must be equal to a reference relative distance (at which the rod is in phase with the cutting head.
Governing the speeds of the two conveyors of the garniture tongue is necessary to make filters from dimensionally identical plugs, that is, in order to cut the filters correctly.
Thus, the absolute and/or relative speeds of the two conveyors are governed in real time in order to allow any phase differences between the two rods and between one of the two rods and the cutting head to be compensated.
During the release of the filter groups to the conveyors, the rotating member retains the two filter groups by keeping the suction means on in such a way that the two filter groups gently push—that is, by applying a slight force to—the other filter groups, which have already been placed on the garniture tongue conveyors.
That way, the rotating member compacts the filter groups positioned on the garniture tongue. In other words, it eliminates any gaps, or empty spaces, between the filter plugs in the same groups and forms two uninterrupted rows of filter groups.
One problem with this machine arises if the relative misalignment between the two rods is too high, that is to say, greater than a predetermined value.
In this condition, when the rotating member simultaneously transfers the two filter groups to the conveyors of the garniture tongue, one of the two filter groups being released may excessively compress the other groups already present on the conveyors and fall out of the flute on the rotating member, thus cancelling the effect of retaining the other filter group in the other flute. As a result, one or more filter groups are missing from the filter rod supplied to the garniture tongue and in the worst cases this may even cause a machine shutdown to allow the fault to be corrected.
This is worsened by the fact that the problem occurs relatively frequently because the filter segments supplied to the reservoirs have variable dimensions (typically of the order of a few tenths of a millimeter) on account of production tolerances. As a result, during machine operation, the two filter rods tend to go out of phase with each other and this can only be partly compensated by adjusting the relative speed of the garniture tongue conveyors.